{"id":17668,"date":"2014-10-27T08:12:40","date_gmt":"2014-10-27T13:12:40","guid":{"rendered":"http:\/\/www.realclimate.org\/?p=17668"},"modified":"2014-12-10T02:06:07","modified_gmt":"2014-12-10T07:06:07","slug":"how-do-trees-change-the-climate","status":"publish","type":"post","link":"https:\/\/www.realclimate.org\/index.php\/archives\/2014\/10\/how-do-trees-change-the-climate\/","title":{"rendered":"How do trees change the climate?"},"content":{"rendered":"
\n

Guest commentary from Abby Swann (U. Washington)<\/a><\/em><\/small><\/p>\n

This past month, an op-ed by Nadine Unger<\/a> appeared in the New York Times with the headline \u201cTo save the climate, don\u2019t plant trees<\/a>\u201d.\u00a0 The author\u2019s main argument is that UN programs to address climate change by planting trees or preserving existing forests are \u201chigh risk\u201d and a \u201cbad bet\u201d. [Ed. There is more background on the op-ed here<\/a>]<\/em><\/p>\n

However, I don’t think that these conclusions are supported by the science.\u00a0 The author connects unrelated issues about trees, conflates what we know about trees from different latitudes, and fails to convey the main point: tropical trees keep climate cool locally, help keep rainfall rates high, and have innumerable non-climate benefits including maintaining habitat and supporting biodiversity.<\/p>\n

A translation of this post in German is available here<\/a><\/lang_de> <\/p>\n

<\/p>\n

Numerous scientists have already replied<\/a> to the original op-ed, highlighting the points above and adding others.\u00a0 But some<\/a> of those<\/a> responses made confusing arguments too, muddying things further.<\/p>\n

So what is going on?\u00a0 Why is it so complicated to say scientifically what trees do to climate?\u00a0 The answer lies in the fact that trees have multiple pathways for influencing climate, and the relative importance of these pathways varies depending on where we look on the globe.<\/p>\n

<\/center><\/p>\n

First lets talk about how plants can influence climate\u2014directly through carbon, directly through energy fluxes, and indirectly through various channels.\u00a0 Then I\u2019ll cover how these factors combine in different places across the globe.<\/p>\n

1. Carbon<\/b><\/p>\n

Carbon dioxide (CO2<\/sub>) in the atmosphere is a greenhouse gas. All plants take in CO2<\/sub> when they photosynthesize, fixing it into sugar and ultimately into the plant tissue itself.\u00a0 At the same time, soils release CO2<\/sub> through decomposition and plants respire it in order to maintain tissues.\u00a0 The two numbers – CO2<\/sub> leaving the atmosphere through photosynthesis and CO2<\/sub> entering it through respiration – are both large and close in magnitude, with slightly more leaving the air than entering.\u00a0 Changes in this delicate balance are one way that the land surface influences global temperature.\u00a0 Planting new trees tips the balance further toward carbon going into the land, at least for a while.\u00a0 When the forest matures, growth and decomposition come back into balance.<\/p>\n

CO2<\/sub> gets mixed quickly by the atmosphere. So after 1-2 years, a release or uptake of CO2<\/sub> anywhere affects CO2<\/sub> everywhere.\u00a0 Right now, plants on average take C02 out of the air (helping keep Earth cool), and it doesn\u2019t really matter where those plants live.<\/p>\n

2. Energy<\/b><\/p>\n

Energy is received by the Earth\u2019s surface directly from the sun and also via long-wave radiation from the atmosphere.\u00a0 The land surface has to balance this incoming energy by radiating energy away and through evaporation and conduction of heat.\u00a0 The hotter the surface, the more it radiates and conducts.\u00a0 So if there is a change in evaporation or in the incoming energy, the only way for the surface to balance the budget is to change temperature. There are three main ways that plants influence the amount and type of energy exchanged between the land and the atmosphere:<\/p>\n

Albedo, or plant color:<\/i><\/p>\n

Plants have different colors, which cause them to absorb different amounts of energy from the sun (the term \u201calbedo\u201d refers to the fraction of light reflected, i.e. how \u201cshiny\u201d something is).\u00a0 In general, trees are darker than grasses, which are darker than bare soil.\u00a0 If we start growing trees where there was once grass or bare ground, the surface will absorb more incoming energy from the sun. **All else being equal**, the surface would have a higher temperature.<\/p>\n

Water fluxes:<\/i><\/p>\n

Plants also release water through their leaves (called transpiration) while photosynthesizing. \u00a0Just as your skin cools when your sweat evaporates, transpiration cools leaves. The amount a plant transpires is directly related to how much it photosynthesizes. Plants that photosynthesize a lot also pump out a lot of water, and with it a lot of energy.\u00a0 If we replace less productive plants or bare soil with highly productive plants, \u00a0**all else being equal<\/em>**, the surface will have a cooler temperature.<\/p>\n

Surface roughness:<\/i><\/p>\n

Trees and plants affect how ‘rough’ the surface feels to the winds. The rougher the surface, the more it slows the winds down, and so trees in particular can affect near surface winds, and this affect leads to a small differences in local stability which can have varying (but small) impacts on fluxes of water and energy from the land surface. <\/p>\n

3. Indirect links<\/b><\/p>\n

Atmospheric circulation<\/i><\/p>\n

While changes in temperature caused by changes in CO2<\/sub> occur evenly across the globe, the same is not true for those driven by changes in albedo or water fluxes. These are local affects, concentrated near the place where the change in tree cover has occurred.\u00a0 But if we plant trees all in one area, and that causes a region of heating or cooling, the atmosphere can respond by changing it\u2019s circulation \u2013 and that might have further climate effects in places far away from where the trees were planted.\u00a0 This can even happen on a global scale.\u00a0 Significant changes in tree cover in Eurasia could cause an energy imbalance between the Northern and Southern Hemispheres, shifting the entire global circulation of the atmosphere, including the location of rainfall in the tropics.\u00a0 \u00a0And changes in where it rains in the tropics would impact plants there, and how they are controlling their local energy balance.<\/p>\n

Plant-emitted volatiles<\/i><\/p>\n

The recent op-ed by also discusses how plants can influence climate by altering chemical reactions in the atmosphere.\u00a0 Plants emit volatile compounds, which are tropospheric ozone and aerosol precursors. According to the op-ed author\u2019s calculations, more trees lead to more particles and more ozone, and the combination **generally** makes things warmer because warming from the heat-trapping ozone is bigger than the slight (and very uncertain) cooling from the particles.\u00a0 This piece of the puzzle is still fairly new and is a great topic for further scientific study and discussion. The exact magnitude of these effects is not yet robustly known.<\/p>\n

<\/center><\/p>\n

4. How does this all play out at different latitudes?<\/b><\/p>\n

In high latitudes, trees are dark and absorb a lot of sunlight, especially in the snowy season when they are much darker than the bright ground below. Also, most high-latitude trees don\u2019t photosynthesize quickly (it\u2019s a tough life up there!). So the water losses and energy fluxes associated with photosynthesis are low.\u00a0 Taken together, this means that Arctic forests help keep the surface warm.\u00a0 As a slight variant on this, some deciduous trees that grow at high latitudes are not quite so dark and have bigger water fluxes.\u00a0 Some of my work suggests that these trees also warm the surface, both because they make the land surface darker and because the water they release acts locally as a heat trapping gas. Trees (and shrubs) also reduce the impact of snow on albedo by standing taller than the snow cover.<\/p>\n

In the tropics, trees photosynthesize at very high rates (growing conditions are great!).\u00a0 These trees shed a lot of water during photosynthesis, with associated high energy losses. \u00a0And tropical trees are not much darker than the pasture grasses that replace them after deforestation.\u00a0 This means that tropical forests help keep the surface cool and wet.<\/p>\n

In the mid-latitudes, both of these effects (albedo and water) are competing. Forests are both darker and have higher water losses than grasses that are likely to replace them.\u00a0 That makes it much harder to say what will happen, on balance, if we were to plant large areas of new mid-latitude trees. The net outcome depends on exactly how much each factor changes, as well as on the amount of moisture in the soil.<\/p>\n

Bottom Line<\/b><\/p>\n

So should we try to slow global warming by planting trees?\u00a0 On a 20-40 year time scale, there is no question that planting trees will transfer carbon from the atmosphere into the trees, slowing the growth of CO2<\/sub> in the atmosphere and thereby slowing global warming.\u00a0 On a 100-year time scale, I would say that we cannot plant our way out of the problem.\u00a0 However, we know that tropical forests keep carbon out of the atmosphere, keep the land surface cool, and play a critical role in providing habitat, maintaining biodiversity, and other good stuff for people.\u00a0 These things are hugely important and it is a no-brainer that we need to fight to keep tropical forests as intact as possible.\u00a0 Maintaining tropical forest does lots of great things, and also helps to slow global warming. But we probably shouldn\u2019t expect to combat global warming in the long term by planting trees in other latitudes.<\/p>\n

It might not totally save the planet, but we should do everything we can to maintain the tropical forest.<\/p>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

Guest commentary from Abby Swann (U. Washington) This past month, an op-ed by Nadine Unger appeared in the New York Times with the headline \u201cTo save the climate, don\u2019t plant trees\u201d.\u00a0 The author\u2019s main argument is that UN programs to address climate change by planting trees or preserving existing forests are \u201chigh risk\u201d and a […]<\/p>\n","protected":false},"author":12,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_exactmetrics_skip_tracking":false,"_exactmetrics_sitenote_active":false,"_exactmetrics_sitenote_note":"","_exactmetrics_sitenote_category":0,"_genesis_hide_title":false,"_genesis_hide_breadcrumbs":false,"_genesis_hide_singular_image":false,"_genesis_hide_footer_widgets":false,"_genesis_custom_body_class":"","_genesis_custom_post_class":"","_genesis_layout":"","footnotes":""},"categories":[44,1,3],"tags":[],"class_list":{"0":"post-17668","1":"post","2":"type-post","3":"status-publish","4":"format-standard","6":"category-carbon-cycle","7":"category-climate-science","8":"category-greenhouse-gases","9":"entry"},"aioseo_notices":[],"post_mailing_queue_ids":[],"_links":{"self":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/17668","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/comments?post=17668"}],"version-history":[{"count":10,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/17668\/revisions"}],"predecessor-version":[{"id":17866,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/posts\/17668\/revisions\/17866"}],"wp:attachment":[{"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/media?parent=17668"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/categories?post=17668"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.realclimate.org\/index.php\/wp-json\/wp\/v2\/tags?post=17668"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}